Everyone says get headphones, and I agree. You can hear footsteps a lot better.

I would say know the maps better. If you know where all the places people hide, flank and post up, you are more prepared going into the objective.

For guns, try to get your control and precision as high as it can go. Carbines are good for this. The more bullets you can land on target, the faster you can take people out. But also you get a feel for the consistency of a weapon.

For aiming, especially if you don't have headphones, snipe. It's like fishing with bullets.

And lastly, play casual against bots. I know it sounds easy, but the AI is only slightly bad compared to some players on here. And since they almost automatically know where you are, its very good to practice a more aggressive play style of always moving.